1. Field of the Invention
The invention relates to a method for generating a scanning clock signal for scanning an analog signal for an analog-to-digital converter operating according to the sigma-delta method, and also relates to an apparatus for carrying out the method.
2. Description of the Prior Art
Such methods and arrangements are suitable when converting analog signals into digital signals, for example in an apparatus for the monitoring, control and regulation of an administration and maintenance unit of a building.
The administration and maintenance unit is for example a heating, ventilation and air-conditioning unit, with access and fire-monitoring equipment or generally a building automation unit or a building management system. In particular, room-climate variables such as room temperature, air humidity and air quality are also influenced by the administration and maintenance unit. In the administration and maintenance unit, a large number of so-called field devices such as sensors and servo elements are to be operated by electric signals. As a rule, analog signals of the field devices are converted into digital signals by analog-to-digital converters for transmission and/or for processing.
Typically, an analog signal is unfortunately also overlaid by an interference variable which is modulated onto the analog useful signal by electric and/or magnetic fields of the power supply network or by components connected to the power supply system. The analog useful signal is for example the signal, corresponding to a measured physical variable, of a sensor; for example of a temperature or moisture sensor. The said interference variable has the frequency of the power supply system and is also called mains hum. The mains hum is often not negligible, in particular in the case of analog signals in a building.
When an analog signal is converted into a digital signal, analog-to-digital converters are widely used which operate by the so-called sigma-delta method. If, in the case of the sigma-delta method, the scanning rate used during the scanning of an analog signal and the decimation rate are suitably chosen, it is known that the sigma-delta method can also act as a filter for certain frequencies of the analog signal. However, an adequate suppression of mains hum can be achieved only if the quotient formed from the scanning rate and from the frequency of the power supply system is constant. As a rule, the scanning rate is derived from an oscillator stabilized with a quartz.
The frequency of the power supply system is 50 Hz in certain regions and countries, but 60 Hz in others. Moreover, the frequency can deviate from a rated value depending on the quality of the power supply system. In the present case a deviation of +/−4% from the rated value is expected.
Without special measures, an adequate suppression of mains hum during an analog-to-digital conversion can be achieved only for either 50-Hz or 60-Hz systems, which stands in the way of a product able to be used world-wide.